1. Field of the Invention
The present invention generally relates to a vehicle air handling system. More specifically, the present invention relates to a vehicle air handling system that maintains the moisture level inside a vehicle passenger compartment within a prescribed range by controlling a mix of fresh air and recirculated air entering the passenger compartment.
2. Background Information
In many vehicles energy consumption is of great concern. For example, there is a continuing need to reduce the amount of energy used by an air conditioning system to improve the energy consumption of vehicles. In summer months, rapid cooling by air conditioning systems is often achieved by recirculating cabin air. After a short cool down period, recirculated cabin air is already cooled and the continued re-cooling of this air reduces the energy burden on the air conditioning system. By contrast, when fresh air is directed into an air conditioning system, cooled air from within the passenger compartment is exhausted from the passenger compartment so that the fresh air can enter in its place. However, cooling fresh air consumes a greater amount of energy than maintaining recirculated air in a cooled state, thus reducing the opportunity for possible power savings.
Similarly, heating fresh air in vehicles, in particular electric vehicles, where all energy is provided by a battery or batteries, reduces the opportunity for power savings that can be realized by utilizing a greater amount of recirculated air. When a fresh air source is heated, its airflow into the passenger compartment displaces previously heated air from the passenger compartment to the outside. The benefits of heating recirculated air can be significant, especially in electric vehicles or hybrid vehicles.
Unfortunately, use of recirculated air in either heating or cooling conditions allows for a build-up of passenger compartment moisture when coupled with passenger respiration/perspiration (or other moisture sources, i.e. wet floors). This is particularly problematic when the build-up of condensation causes moisture to form on interior window surfaces (fogging) or creates an unpleasant or “muggy” sense of human discomfort.
In some vehicles, a dew point temperature sensor is provided on an interior side of the windshield and serves to acquire the dew point based on a temperature and relative humidity of air flowing near the surface of the windshield. The dew point is a temperature at which water vapor contained in air will start to condense when the air is cooled. More specifically, when water vapor is cooled to the dew point temperature the water vapor becomes saturated. Relative humidity is obtained by dividing an amount of water vapor contained in ambient air at a given temperature by a saturated water vapor amount corresponding to that temperature. For example, a relative humidity of 100% reflects a condition where the ambient air is saturated with water vapor and condensation will occur. Thus, the dew point is a temperature at which the relative humidity is 100%.
Since condensation on the interior of vehicle glass can frustrate occupants and potentially impair the ability of occupants to see outside of the vehicles, it is advantageous to maintain moisture levels within a vehicle below the dew point.